HYDRAULICS. 207 



tea-kettle that boils. Steam itself is transparent, 

 and consequently invisible, and it is only by being 

 condensed again into water, that it assumes the 

 misty form by which we see it. The steam from 

 the spout of a tea-kettle is transparent and invi- 

 sible just as it issues, and does not become visible, 

 till at a small distance from the aperture ; it being 

 soon condensed by the atmosphere. 



Steam at the temperature of 212° is a perma- 

 nent vapour, possessed of great elasticity, and 

 capable of exerting a prodigious force when con- 

 fined in close vessels. It can also be heated to a 

 much higher temperature, by which its elasticity 

 and power is still farther encreased ; and the de- 

 gree of temperature is the exact measure of its 

 elasticity. 



If steam be thrown into a vessel already full of 

 air, it will drive out the air and occupy its place : 

 if then, by the application of cold, the steam 

 within the vessel be condensed into water, a 

 vacuum will be produced in the vessel, since the 

 space occupied by the condensed water is ex- 

 tremely small compared to that which was filled 

 by the steam. 



The steam-engine is one of the noblest monu- 

 ments of human ingenuity. It was originally in- 

 vented by the Marquis of Worcester, in the reign 

 of Charles II. This nobleman published, in 1663, 

 a small book called " A Century of Inventions" 

 describing a hundred discoveries or contrivances 

 of his own ; but the descriptions of many of them 

 are so obscure, that they are altogether unintel- 

 ligible. 



Among them is an account of his invention of 

 raising water by the force of steam, which, now 

 that we are possessed of the engine, appears to 



